Relative permeability.

Relative permeability, sometimes denoted by the
symbol μr , is the ratio of the permeability of a
specific medium to the permeability of free
space, μ0:
where μ 0 = 4π × 10 −7 N A −2. In terms of
relative permeability, the magnetic susceptibility
is
χm , a dimensionless quantity, is sometimes
called volumetric or bulk susceptibility, to
distinguish it from χp ( magnetic mass or
specific susceptibility) and χM (molar or molar
mass susceptibility).